1. Field Of The Invention
The present invention relates generally to the treatment of a subterranean formation with sequential crosslinked gelled fluids having substantially different acid/base properties. More particularly, the present invention relates to the use of an acidic delayed crosslinking gel which may have a gas present, such as nitrogen or carbon dioxide, to initially create a fracture and a second alkaline, reversible crosslinked gel to enhance fracture growth and maximize proppant introduction into the created fracture.
2. Description Of The Prior Art
Numerous subterranean formation treatments have been developed utilizing crosslinked aqueous gels. For example, a well known technique for stimulating a subterranean formation involves fracturing the formation with a viscous crosslinked aqueous gel. In general, the aqueous gel is injected down a wellbore penetrating the formation at a rate and pressure sufficient to develop hydraulic forces on the formation and create fractures therein. Continued pumping of the fluid results in extension of the fractures. Particulates, known as propping agents, are added to the gel to be deposited in the created fracture and prop the fracture open at the conclusion of the treatment to enhance subsequent hydrocarbon production from the subterranean formation.
In order to avoid the difficulties associated with pumping crosslinked gelled fluids from the surface to a subterranean formation, that is, high surface pressures and horsepower requirements for the pumps, delayed crosslinked gelled fluids have been developed. The delayed crosslinking gelled fluids utilize crosslinking agents that effect a delay between the time of addition to the gelled aqueous fluid and the onset of crosslinking of the aqueous gel. The delay is generally controlled to permit pumping of the gelled fluid into the wellbore from the surface before crosslinking occurs, thereby minimizing problems associated with pumping highly viscous fluids. Such processes are disclosed in, for example, U.S. Pat. No. 4,619,776, the entire disclosure of which is incorporated by reference.
In some applications, gases such as nitrogen or carbon dioxide are combined with the fluid. The purpose of the gas is to reduce the water content of the fluid and energize the return of the fluid following the treatment. The gas can be present in an amount of from about 5% to in excess of 90 percent. When carbon dioxide is utilized, it may be added to the fluid as a liquid and be permitted to convert to a gas in the formation or wellbore. Such processes are disclosed in, for example, U.S. Pat. No's. Re 32,302, 4,488,975 and 4,799,550, the entire disclosures of which are incorporated by reference.
For optimum production results from a treated well it is advantageous to remove as much of the treating fluid as possible after the treatment is concluded.
By the present invention, an improved method of treating a subterranean formation with delayed crosslinking aqueous gels is provided which can maximize proppant introduction while maximizing fluid returns and improving well cleanup following the treatment. The present method also provides elevated temperature stability, enhancing the ability of the fluids to effectively transport high concentrations of proppant material into created fractures.